Sizing cables for Star-Delta applications

[bfuchs]

In star-delta starters, there are two cables feeding the motor. Let's call them In and Out cable. In the delta configuration, the current flowing through these cables is Ib/1.732, where Ib is the current drawn through the upstream protection device.<br>To what extent is it permissible to size these cables on this reduced current, and not the current flowing through the breaker ?<br>Standards specify that a conductor must be sized on the thermal protection upstream and not the expected current flowing through the cable.<br><br>Based on this, these cables should therefore always be sized on the setting of the thermal relay. <br><br>If the thermal relay is included in the circuit breaker, then the cables should be rated on the rating of the breaker.<br><br>If the thermal relay is separate and protects the delta cable (and therefore the line cable), it can be chosen to be close to the expected load on these cables. Therefore smaller cables could be used. Unfortunately using smaller relays penalises the coordination between the circuit breaker-relay-contactor (type1 and type2 coordination).<br><br>What solutions do you recommend ?<br>&nbsp;&nbsp;&nbsp;- fully rated cables and relays ?<br>&nbsp;&nbsp;&nbsp;- reduced relays and cables ?<br>&nbsp;&nbsp;&nbsp;- reduced cables even with a thermal protection device included in the circuit breaker ?<br>&nbsp;